A newborn tested positive for toxoplasma IgG. Which of the following can be done to confirm if the newborn is really infected?

A newborn tested positive for toxoplasma IgG. Which of the following can be done to confirm if the newborn is really infected?

• A. Perform IgG from maternal sample
• B. Perform IgM from current newborn sample
• C. Perform IgG from newborn sample
• D. Perform IgA from newborn sample

Answer: (B)

In newborns, maternal antibodies circulating in the baby's blood can cross the placenta, so finding IgG in the infant does not prove infection—the IgG could come from the mother. To confirm congenital infection, you want evidence that the baby has mounted an immune response. IgM is the antibody class produced by the fetus in response to infection, and it does not cross the placenta. Therefore, detecting toxoplasma-specific IgM in the newborn’s serum indicates the baby was infected in utero. IgG from the newborn would not distinguish maternal from fetal antibodies, since maternal IgG can be present in the newborn for months after birth. IgG from the maternal sample also cannot confirm neonatal infection. IgA is not a standard marker used to confirm congenital toxoplasmosis in newborns. So the best approach is testing the newborn for IgM to establish congenital infection.

In newborns, maternal antibodies circulating in the baby's blood can cross the placenta, so finding IgG in the infant does not prove infection—the IgG could come from the mother. To confirm congenital infection, you want evidence that the baby has mounted an immune response. IgM is the antibody class produced by the fetus in response to infection, and it does not cross the placenta. Therefore, detecting toxoplasma-specific IgM in the newborn’s serum indicates the baby was infected in utero.

IgG from the newborn would not distinguish maternal from fetal antibodies, since maternal IgG can be present in the newborn for months after birth. IgG from the maternal sample also cannot confirm neonatal infection. IgA is not a standard marker used to confirm congenital toxoplasmosis in newborns. So the best approach is testing the newborn for IgM to establish congenital infection.